In a Hard Disk Drive (HDD) system having a Magneto-Resistive (MR) head for reading the data on the disk, the resistance of the MR head varies according to the magnetic field applied to the MR head from the disk. A circuit is required to convert the variation in resistance of the MR head into an output signal. One such type of circuit is a current biased and current sensing preamplifier. Such a preamplifier applies a DC bias voltage across the MR head to achieve a predetermined bias current through the MR head and senses a variation in the current as the resistance varies, i.e. as the magnetic bit cells (representing the data) on the disk pass the MR head. The sensed current variations are converted into an output signal representing the data stored on the disk. Therefore, the functions provided by such a preamplifier include:
biasing the MR head with a predetermined current; PA0 sensing the small AC variations in current; and PA0 converting the variations into a relatively large output signal, usually a voltage signal.
Such an amplifier must be capable of operating at high frequencies, since the amount of data that can be stored on the disk depends on the speed at which the data can be read out, whilst generating as little noise as possible.